The present invention relates primarily to a method of charging nickel cadmium and nickel hydrogen rechargeable batteries.
The --.DELTA.V detection method, where rapid charging is stopped when a small voltage drop from the peak battery voltage is detected, is adopted as a charging method for the nickel cadmium battery. This method utilizes the property shown in FIG. 1 that battery voltage drops by a voltage --.DELTA.V from its peak value when the battery becomes fully charged. This method has the weak point that the rechargeable battery can be over-charged when --.DELTA.V cannot be detected normally. --.DELTA.V is difficult to detect when charging current is small with only gradual changes in battery voltage, when battery temperature is low, and when nickel hydrogen batteries are charged. When --.DELTA.V cannot be detected and the battery is over-charged, not only is rechargeable battery performance reduced, but there is also the danger of explosion due to gas pressure build-up within the ease.
To overcome this problem, a method of detecting peak voltage instead of --.DELTA.V is recited in Japanese Non-examined Patent Publication No. 1-177834 issued Jul. 14, 1989. In this disclosure, the rechargeable battery is fully charged by supplementary charging for a set time interval determined by a supplementary charging timer after peak voltage has been detected. This is done because the rechargeable battery has not reached full charge when the peak voltage is detected.
However, when charging conditions differ, it is difficult to charge all rechargeable batteries to full charge without over-charging using a method that detects peak battery voltage during charging and then performs supplementary charging for a fixed interval. This is because rechargeable batteries under different charging conditions will have different charging capacities at the peak voltage detection point.
For example, when rechargeable batteries with different residual battery capacities are charged, they will have different charging capacities at the peak voltage detection point. A completely discharged rechargeable battery that is charged until it reaches peak voltage, will still not be near full charge. On the other hand, a battery near full charge that is charged until it reaches peak voltage, will be in a state close to full charge. Consequently, it is difficult to set the supplementary charging time interval after the peak battery voltage for a method with a fixed supplementary charging interval. If the supplementary charging time interval is set to fully charge a completely discharged battery, a rechargeable battery near full charge will be over-charged. However, if the supplementary charging time interval is reduced to avoid over-charging a battery near full charge, a completely discharged battery will not be fully charged.
Further, rechargeable battery voltage characteristics during charging vary according to the charging current as well as battery capacity. If the rechargeable battery is charged with a large charging current, the peak voltage value will be detected early, and if the charging current is reduced, the peak voltage value will be detected late. Therefore, a battery charged by a small charging current will be in a state close to full charge when the peak battery voltage is detected. However, a battery charged by a large charging current will not be close to full charge when the peak battery voltage is detected. Consequently, as described previously, when supplementary charging is performed for a fixed timer interval after peak voltage detection, all rechargeable batteries cannot be fully charged without over-charging.
The present invention was developed to overcome the above mentioned disadvantages. It is thus a primary object of the present invention to provide a rechargeable battery charging method that can fully charge a rechargeable battery under various charging conditions without over-charging.